The sponge is a group of animals representing a distinct branch or phylum, Porifera. The sponges are many-celled animals, with three-cell layers, without a true digestive cavity, supported usually by calcareous or siliceous spicules, the body mass permeated by ciliated passages or containing minute chambers lined by flagellate, collared, monadlike cells. Sponges are hermaphroditic, multiplying by fertilizing eggs, the germ passing through a cleavage and a gastrula stage.
Sponges are divided, according to the nature of the skeleton or supporting fibers or spicules, into two orders; Calcispongiae, in which the spicules are calcareous, and Silicispongiae, or glass sponges, in which the spicules are siliceous, or horny and fibrous, as in the bath sponges, when spongin instead of silica forms the supporting framework. Examples of the siliceous sponges are the Venus's flower basket, Euplectella, and allied deep-sea forms, Hyalonema and Holtenia. To the second order also belongs the fresh-water sponge, Spongilla, growing in lakes or sluggish streams. These differ from other sponges in producing statoblasts or winter buds, formed by the protoplasm dividing into round bodies as large as a pin's head and enveloped by a dense membrane, thus enabling the species to survive freezing colds or droughts.
Certain sponges bore into shells, causing them to disintegrate. Of the marketable sponges there are six species, with numerous varieties. They are available for use from being simply fibrous, having no siliceous spicules. The Mediterranean sponges are the softest and best; those of the Red Sea are next in quality, while the West Indian species are coarser and less durable. The glove sponge, Spongia tubulifera, corresponds to Spongia adriatica (the Turkey cup sponge and Levant toilet sponge of the Mediterranean). S. gossypina, the wool sponge of Florida and the Bahamas, corresponds to S. equina, horse or bath sponge of the Mediterranean. This wool sponge of Florida attains under favorable circumstances a weight of one-tenth pound, in six months, and reaches a size of commercial value in a year.
